Question

我是Android开发的新手，我尝试在我的（目前为止）活动中创建一个简单的SharedPreferences。但是，每次我尝试运行它时，即使我在声明行上分配变量，我也会在运行时获得Null-Pointer异常。我的全部记录：

java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{andyszoke.macrohub/andyszoke.macrohub.SetMacroManualActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
                                                       at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2988)
                                                       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3242)
                                                       at android.app.ActivityThread.access$1000(ActivityThread.java:205)
                                                       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1695)
                                                       at android.os.Handler.dispatchMessage(Handler.java:102)
                                                       at android.os.Looper.loop(Looper.java:145)
                                                       at android.app.ActivityThread.main(ActivityThread.java:6895)
                                                       at java.lang.reflect.Method.invoke(Native Method)
                                                       at java.lang.reflect.Method.invoke(Method.java:372)
                                                       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1404)
                                                       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199)
                                                    Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
                                                       at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:111)
                                                       at andyszoke.macrohub.SetMacroManualActivity.<init>(SetMacroManualActivity.java:17)
                                                       at java.lang.reflect.Constructor.newInstance(Native Method)
                                                       at java.lang.Class.newInstance(Class.java:1690)
                                                       at android.app.Instrumentation.newActivity(Instrumentation.java:1080)
                                                       at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2978)
                                                       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3242) 
                                                       at android.app.ActivityThread.access$1000(ActivityThread.java:205) 
                                                       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1695) 
                                                       at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                       at android.os.Looper.loop(Looper.java:145) 
                                                       at android.app.ActivityThread.main(ActivityThread.java:6895) 
                                                       at java.lang.reflect.Method.invoke(Native Method) 
                                                       at java.lang.reflect.Method.invoke(Method.java:372) 
                                                       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1404) 
                                                       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199)

以及相关的代码：

package andyszoke.macrohub;

import android.content.Context;
import android.content.SharedPreferences;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.app.WindowDecorActionBar;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class SetMacroManualActivity extends AppCompatActivity {
    private static final String TAG = "myMessage";
    private static final String MY_PREF = "MyPreferences";
    SharedPreferences prefs = this.getSharedPreferences(MY_PREF, 0);
    SharedPreferences.Editor editor = prefs.edit();

在类的第三行抛出异常。我已经尝试了各种不同的声明组合，并查看了尽可能多的stackoverflow帖子，但没有任何东西可以解决它。任何帮助将不胜感激！

Answer 1

用于设置共享首选项的代码在onCreate之前运行。出于所有常规目的，只应在onCreate之前分配变量。当你说过这个.getSharedPreferences时，这个词就是这个＆＃39;是指活动，但尚未调用onCreate（因此活动尚未形成）。所以＆＃39;这个＆＃39;被分配给空对象。所以获得了NullPointerException。

public class SetMacroManualActivity extends AppCompatActivity {
  private static final String TAG = "myMessage";
  private static final String MY_PREF = "MyPreferences";
  @Override
  public void onCreate(Bundle savedInstanceState)
  {
    super.onCreate(savedInstanceState);
    // load the layout
    setContentView(R.layout.filters); 
    SharedPreferences prefs = getSharedPreferences(MY_PREF, MODE_PRIVATE);
    SharedPreferences.Editor editor = prefs.edit();

MODE_PRIVATE是通常使用的，还有其他基于要求的模式。所以请检查一下对你来说没问题。

Android在实例化SharedPreferences

1 个答案: